From US Centers for Disease Control (2004) (U.S. Data) and  Boyce et al. (2003)(Canadian Data)

           % of high school students who have

                had sexual intercourse

                     US              Canada

                Male    Female    Male    Female

Grade

9                37%      28%     23%      19%

10               45%      43%    

11               53%      53%     40%      46%

12               61%      62%

From Hofferth (1990):  Timing of second intercourse among 15-19 year old, sexually experienced women

  Within 1 month               41%

  1-3 months                   24%

  4-6 months                   13%

  7-12 months                    7%

  >12 months                     7%

  no second time                 6%